1. Field of the Invention
The present invention relates to plate alumina particles having high dispersibility and plasticity and suitable for use as a base of pigments, particularly pearl pigments, for paints; extender pigments for cosmetics; a precision abrasive; a starting material for ceramics, etc. The present invention relates also to a process for efficiently producing the plate alumina particles by a hydrothermal synthesis treatment.
2. Description of the Prior Art
Alumina has been widely used hitherto on an industrial scale as a typical material for ceramics as described below. Although various properties are required of alumina depending on the use thereof, alumina particles produced hitherto not always satisfied these requirements.
For example, when alumina is used as a pigment for a paint or as a precision abrasive, it is used mostly in the form of an aqueous slurry thereof so as to homogenize the particles in the step of producing the starting material or mixing with another material. In such a case, the starting powder must be completely dispersed to primary particles in order to exhibit the properties and function of the starting material to the maximum.
As is well known, the plate particles exhibit masking properties and particle orientation superior to those of granular particles when they are used as a material for a pigment. Thus, the plate particles are desirable particularly for use as the base of pearl pigments. However, these properties cannot be obtained when the particles are in an agglomerate form. Therefore, the powder is freed from the agglomeration by pulverization or cracking in the presence of a dispersant with a wet-type ball mill or the like for a long period of time.
On the other hand, when .alpha.-alumina is used as a material for ceramics, particularly as a material for plastic molding, its plasticity is lower than that of a natural high-quality clay. Such a problem is solved mostly by mixing or incorporating an organic matter or a binder having a water retentivity as a molding assistant.
A fundamental requirement for realizing the coloring function is that the primary particles are stably dispersed as pigment particles in the paint thus obtained. When the pigment particles agglomerate due to insufficiency in stability, color segregation, color shift and pigment sedimentation are caused.
When the pigment sediments, the resultant agglomerate contains the vehicle and fixes it therein to seriously increase the apparent volume of the pigment and also to increase the viscosity. In such a case, the agglomerate structure is broken by a shearing force in the coating step to significantly change the flow properties of the paint by the shearing force. The dependency of the flow properties on the shearing causes a serious reduction in workability in the coating step.
Also when alumina particles are used as the abrasive, the agglomerate particles cause reduction in the abrading properties and particularly in the polishing step which is the final step in the abrasion, the lubricity and specular surface properties of the finished surface are seriously impaired. When alumina particles are used as a plastic molding material for ceramics, an organic substance is also used as the molding assistant. In such a case, the adhesion of the blended material is too high disadvantageously in a high water content zone in the plasticization water content range. Thus, only a low water content zone in the plasticization water content range is usable and, therefore, the molding pressure is made high thereby to limit the molding machine used and the like disadvantageously. It was considered that organic substances remaining undecomposed in the sintering step make the production of dense alumina ceramics by the plasticization molding difficult.
Under these circumstances, an alumina material capable of keeping its shape without using any molding assistant and having only a low adhesion like a natural clay is eagerly demanded.
Various processes are known hitherto for the production of alumina particles. Particularly, investigations on the production of highly pure alumina particles are remarkable. These processes are typified by, for example, thermal decomposition of ammonium alum, hydrolysis of an organometal, ethylene chlorohydrin process, spark discharge of aluminum in water, ammonium aluminum carbonate heat-decomposition (AACH), modified Bayer's process and gas phase oxidation process. For the production of alumina particles on a large industrial scale, ordinary Bayer's process is most common. Although the processes for producing highly pure alumina are characterized in that the alumina particles thus synthesized has a purity of as high as 4N or 5N, these processes have a problem that the cost must be reduced. On the other hand, Bayer's process is characterized by a low cost, since it is a mass production process. However, this process also has problems that the production of the highly pure product is difficult, that a mechanical means such as a ball mill or medium stirrer mill is necessitated for obtaining the fine particles and that the particles are contaminated in the pulverization step. Although the alumina particles produced by the above-described processes are generally very fine, their shapes are inclined to be granular or irregular.
For the production of the plate alumina particles, a process wherein a mineralizer such as aluminum fluoride is added in a step of calcination of the starting material as described in, for example, Japanese Patent Publication No. 6977/1960 is known. Further, for the production of alumina particles by a hydrothermal synthesis method, a process described in, for example, Japanese Patent Publication No. 7750/1962 is known.
However, when the plate alumina particles are produced by the above-described process with a mineralizer (Japanese Patent Publication No. 6977/1960) or the hydrothermal synthesis method (Japanese Patent Publication No. 7750/1962), the control of the size of the alumina particles thus produced is difficult and, particularly, the control of the thickness of the plate particles for thinning them is very difficult. When plate particles having such a low aspect ratio (particle size/thickness of plate particle) are used as a base for pigments, particularly pearl pigments, for paints or as extender pigments for cosmetics or the like, the orientation of the particles is poor; and in the former, the light reflection is nonuniform to make the realization of pearly feeling difficult and in the latter wherein the plate particles are used as the extender pigments for cosmetics or the like, the adhesion to the skin and spreading become poor. Thus, the problem is that the essential properties of the plate particles are difficultly realized.
Even when the plate alumina particles are obtained, the dispersibility and plasticity of them are actually insufficient for obtaining satisfactory results.